In many applications in the field of recording and archival storage medium, e.g., direct-read-after-write archival storage medium for recording high density data and high quality images in particular, a recording life time for data substantially longer than ten years is desirable. It is also desirable to produce phototools, such as photomasks and reticles, that are substantially more durable than chrome-phototools.
These objectives may be achieved by having the recorded image and/or the masking pattern and/or the information bearing layer present within (that is, not coated on) the glass substrate of the recording medium. Such a recording medium would improve the durability of the phototool and the recorded image and eliminate the need of thin film coatings, such as the chromium film on photomasks and ablative thin film materials, e.g. Tellurium film on optical disks.
There are many types of glasses in which optical images can be produced within the glass, such as the glasses of the following patents:
U.S. Pat. No. 4,017,318 discloses photosensitive colored glasses which are sodium halide-silver halide containing photosensitive glasses based on sodium silicate. The photosensitive colored glasses contain a significant amount of fluoride, 1-4% by weight, and a second halide, usually about 1% by weight of Br. On cooling from melts, the glasses become supersaturated with NaF. They also contain 0.0005-0.1% by weight of Ag as sensitizer and colorant, 0.01-0.2% CeO.sub.2 as optical sensitizer, about 1% Sb.sub.2 O.sub.3 and/or up to about 0.1% SnO as thermal sensitizer and redox agent.
Through a unique sequence of ultraviolet exposures and heat treatments, a full spectrum of visible colors can be developed in a photosensitive colored glass of U.S. Pat. No. 4,017,318. While the intensity of colors depends on the exposure dosage of a second uv exposure, the hue is primarily a function of the first uv exposure dose. Thus, optical transmission curves for a fully developed glass show a progressive series of absorption bands moving from the infrared across the visible spectrum as a function of the increasing dosage of first ultraviolet exposures. These photosensitive colored glasses are often yellow colored in the regions/areas which are not exposed to ultraviolet radiation, after a sequence of heat treatments. In order to prevent the spontaneous yellow background coloration, the silver concentration in the photosensitive colored glass melts of U.S. Pat. No. 4,017,318 is kept below 0.025% by weight. As a result, the required thickness of the image bearing glass layer is at least about 100 micrometers to secure an optical density of unity. Thus these glasses are not suitable for use as a recording medium to store data having bit sizes of less than about 10 micrometers or as photomasks to print IC patterns having minimum geometries of less than about 10 micrometers.
U.S. Pat. No. 2,515,936 discloses silver-photosensitive glasses which are exposed to short wave radiation, such as ultraviolet (310 nm). The irradiated areas of the glass are capable of heat developed coloration while non-irradiated areas remain substantially unchanged on heating. The heat developed image is permanent and is yellow to amber in color due to an absorption band centered at about 420 nm. The glass batches on the weight percent basis contain about 75% SiO.sub.2, 15% alkali metal oxide and 10% divalent metal oxide, 0.05 to 0.3% silver computed as AgCl, and up to 0.05% CeO.sub.2. CeO.sub.2 is stated to act as a photosensitizing agent. Although up to 0.3% of silver can be retained in the glass melt, the thickness of the silver-photosensitive glass, required to secure 1 unit of optical density at 420 nm, is not less than that of the photosensitive colored glasses of U.S. Pat. No. 4,017,318. Apparently a smaller fraction of silver ions in the silver-photosensitive glass of U.S. Pat. No. 2,515,936 is reduced and precipitated out as colloidal silver metal particles in glass as a result of a radiation step followed by a heat treatment.
U.S. Pat. No. 2,732,298 discloses a method of producing a permanent two dimensional photographic image within the surface of a silicate glass containing an alkali metal oxide. The method consists of first forming on the surface of a silicate glass by conventional procedures a chemically fixed photographic image. The glass, together with the image, is then heated either in air alone or with an additional oxidizing agent present at a temperature between about 125.degree. C. below the strain point of the glass and just below its softening point for a time sufficient to oxidize and ionize the silver and to cause the migration of silver ions into the glass in exchange for alkali metal ions therein. A visible image which is usually of a yellowish to brownish color is produced due to the spontaneous reduction of a portion of the silver ions in the glass to metallic particles. If desired, a more complete reduction of silver ions and intensification of the color to dark amber or brown can be accomplished by subsequently heating the glass in a reducing atmosphere containing a reducing gas such as hydrogen, carbon monoxide or methane. Due to the lateral or sideway diffusion of the photographic silver image, the minimum geometry and/or line width reproducible in glass according to this method is in general greater than about 20 micrometers.
A modification of the method of U.S. Pat. No. 2,732,298 is found in U.S. Pat. No. 4,286,052 wherein the photographic silver image is driven into a glass article by electrical means.
Besides being not suitable for recording optical images or information with bit sizes or minimum geometries of less than a couple of micrometers, images in the three types of glasses cited above are developed through at least a heat treatment step, there is thus no direct-read-after-write capability.
Applicant's U.S. Pat. Nos. 4,191,547 and 4,296,479 are directed to alterable photosensitive glasses, that is, photosensitive colored glasses exhibiting alterable photo-anisotropic effects. These glasses may be employed as erasable recording media having limited read applications. The alterable photosensitive glasses comprise a body portion and a photosensitive surface layer exhibiting alterable photo-anisotropic effects. The surface layer, having photosensitive and photo-anisotropic Ag-AgCl-containing crystals therein, is prepared by subjecting a glass article having base glass compositions within the Na.sub.2 O and/or K.sub.2 O-ZnO-Al.sub.2 O.sub.3 -SiO.sub.2 -Cl field to an ion-exchange reaction in a silver containing aqueous solution, and subsequently exposing the glass article to radiation within the spectral ranges of uv and blue light. Write and erasure are done with polarized red light, and are based on the phenomena of photo-polarization and optical bleaching.
The alterable photosensitive glasses of U.S. Pat. Nos. 4,191,547 and 4,296,479 are not suitable for use as a permanent recording medium and/or as a phototool because there is no method and/or mechanism for fixing recorded images. The recorded image, as well as the alterable photo-anisotropic surface layer, are unstable in the presence of radiation in the uv, blue light and longer wavelength range, and are particularly sensitive to polarized red light. The required thickness of the image/information bearing layer of the alterable photosensitive glasses to secure an optical density of 2 is at least 30 micrometers.